Preheat oven to 150 C degrees.
Line standard muffin tins with paper cases/cupcake wrappers.
Melt butter and chocolate in a large heatproof bowl set over (not in) a pan of simmering water. Stir to combine, then remove the bowl from heat and let cool slightly.
Whisk egg yolks into the chocolate mixture.
With an electric mixer on medium speed, whisk egg whites, with the pinch of salt, until soft peaks form. Gradually add sugar, beating until peaks are stiff and glossy but not dry (do not overbeat). Whisk one-quarter of the beaten egg whites into the chocolate mixture to lighten. Then, gently fold the mixture into the remaining egg whites.
Divide batter evenly among lined cups, filling each three-quarters full. Bake for 25 minutes, rotating tins halfway through. The cakes should just be set in centres.
Whilst the cakes are still hot, add a chunk of the remaining chocolate in the centre of each cake by gently pushing it down to nestle in the centre of each cake. Cool the pan on a rack for 10 minutes and then unmold them. Transfer tins to wire racks to cool completely before removing cupcakes. Because these cakes are flourless, their centres may sink a little and crack.
Dust with icing sugar or rich dark cocoa.